The present invention relates to improvements on the electron optical system of an environmental scanning electron microscope suitable for observation of uncooked food or moist specimens.
In a scanning electron microscope (SEM) being suitable for observation of specimens in natural condition such as uncooked food or moist specimens and called an environmental or low-vacuum SEM, an electron gun chamber is maintained at high vacuum, whereas a specimen chamber is maintained at low vacuum approximating natural condition. Specifically, the electron gun chamber included in an electron optical system is at 10−2 Pa to 10−4 Pa but the specimen chamber is at several Pa to several of hundreds of Pa.
U.S. Pat. Nos. 5,362,964 and 5,412,211 disclose an environmental SEM having two orifices and a cylindrical electron beam path in order to be constructed such that differential pumping is effected through substantially three steps of evacuation conductance to assure a large pressure ratio between the electron optical system and the specimen chamber.
And also, JP-A-1-183047 discloses an SEM capable of observing specimens in natural condition, which is constructed to have three stages of orifices for differential pumping.
In the construction disclosed in the aforementioned US Patents, however, the deflection area on the surface of a specimen is restricted by the disposition of the cylindrically extensional pressure orifice. Further, the distance from an excitation point of objective lens to the specimen surface is long and disadvantageously, the resolution the SEM can exhibit is degraded.
In the case of JP-A-1-183047, the deflection area on the specimen surface is likewise liable to be restricted as gathering from the illustrated construction and besides, the specimen surface is situated remotely from an excitation point of objective lens and the resolution the SEM exhibits is considered to be hardly improved.